This invention relates to all optical signal processing devices.
Optical fibers have a signal carrying bandwidth that potentially is in the multi-terabit range. This makes optical fibers an attractive choice for signal transmission systems, particularly in long haul transmission systems where the cost of the cable makes up a large fraction of the cost of the communications link. Fiber is far superior to any competing technology in terms of bandwidth, cable size and other factors. However, signal attenuation in long fiber segments is still an issue that has to be addressed. To compensate for signal losses, current optical fiber transmission systems employ electronic repeaters at spacings on the order of tens of kilometers. To detect, re-synchronize, and regenerate the signal, the repeater must convert the optical signal to electrical form, amplify it, and reconvert the amplified signal to optical form. Aside from the complexity and expense associated with such conversions, the need to work in the electronic domain is limiting the bandwidth of the overall transmission system.
It is expected that electronic regeneration will be replaced by fiber amplifiers, which have an overall information-carrying capacity comparable to that of the fiber itself. Erbium fiber amplifiers represent a new technology and promise to provide better performance at a lower cost. While amplifiers can compensate for fiber attenuation by boosting the signal level, the functions of signal regeneration and signal re-timing are not addressed by the amplifier. Without these functions there is no means of restoring the data, and any noise which appears at any point in this type of system merely accumulates.
Moreover, broadband noise over the entire gain bandwidth of the amplifier (known as amplified spontaneous emission) is an inevitable by-product of the amplification process. Consequently, the amplifiers themselves are a significant source of noise in the system. Because the signal itself does not generally occupy the entire amplifier bandwidth, a spectral filter can be used to stop all noise outside the signal bandwidth. While this is an important step in reducing the bit error rate at the receiver, it does nothing to the in-band noise; and it is that noise which generally causes the most errors.
An approach for reducing the noise in the baseline of an optical signal is described in "Pulse shaping, compression and pedestal suppression employing a nonlinear-optical loop mirror", Doran et al., Optics Letters, Vol. 15 No. 22, Nov. 15, 1990, pp. 1294-1296, where the authors reported on the signal transmission properties of a sagnac interferometer. They note that the input/output transfer function 10 of such an interferometer is oscillating between peaks and troughs (see FIG. 1), and that the oscillating function is bounded by two lines: one is a 45 degree line starting at the origin (line 11), and the other is by a nearly horizontal line (line 12). They also note that by using only that portion of the input/output transfer function that starts at the trough next to zero input and ends at a relatively linear portion of the transfer function, the baseline of the signal is compressed relative to the rest of the signal. Any noise that rides on top of that baseline would be compressed.
The fact that the slope of the input/output transfer function starts at a low value and increases as the input power increases also causes the output pulse to be narrower than the input pulse. Doran et al. note that this phenomenon may be thought of as pulse shaping in the form of temporal pulse compression. Increasing the signal into the interferometer much further first shortens the tips of the pulses and generates substantial distortion of the pulses.